Hi,
You could combine a custom field of the table "user" with a mass action configured like this:
- a trigger "when a user is created"
- an action "update the values" on the column of the custom user field, with the mode "operation" and a value like this:
concat('xxx-',user.user_id)
This would create a user number with the format xxx-yyy
where xxx is the prefix you entered in the mass action, and yyy the id of the user_id.
The format can be tweaked as needed in the value of the action in the mass action.
While this requires a bit of setup, it seems simple enough for something that looks niche enough that you're the first one talking about it in 15 years on this forum as far as I know. So I don't think it warrants the development of that capability by default with yet another option by default in HikaShop.